Locks provide fairly simple devices that allow boats to go up and down from one level to another within a canal. A schematic representation of a canal lock 1 is presented in FIG. 1. The lock essentially consists of a watertight chamber 2, two gate sills 3 and two Mitre gates 4 located at either end of the chamber 2, themselves comprising of two gate leaves 5. Sluice valves 6, sluice chambers 7 and water valves 8 are incorporated within these Mitre gates 4 to allow the water to enter into the chamber 2, filing it, or allowing water out of the chamber 2 draining it. Associated with each Mitre gate 4 is a hand winch mechanism or a hydraulic cylinder 9, a gate sluice control 10 and a gate recess 11.
As is well known in the art the systematic control of the position of the sluice valves 6, sluice chambers 7 and water valves 8 allow for a boat 12 to move through the lock 1 in either direction as desired. The same Mitre gate 4 systems are also traditionally employed as watertight gates for ports.
In order to operate correctly the Mitre gates 4 require to be watertight. This is achieved through the employment of a heel post, Mitre post and seals as are well known in the art. Hydraulic pressure that is a direct result of the difference in the water levels from the upper side and lower side of the Mitre gate 4, acts to maintain the watertight seal. The hydraulic pressure forces the lower side of the gate leaves 5 against the gate sill 3 forming a seal while the Mitre post provides the required seal between the gates leaves 5.
Traditionally the heel post, Mitre post and seals are constructed using hardwood such as greenheart or ekki, as these are resistant to marine borers and other forms of decay. As well as providing the sealing means, the heel post and the Mitre post also act as the load bearing components for the structure.
Two major disadvantages have developed with the use of the aforementioned Mitre gates 4. The first is a result of the dependency for hydraulic pressure to maintain the gates 4 in the closed watertight position. Water levels are not consistent all year round and more significantly global warming has seen average water levels rise over recent years. Therefore, the efficiency of the Mitre gate 4 is seasonally dependent and in general the overall efficiency is being reduced due to global warming effects.
Attempts to provide a solution to this reverse head of water pressure include the incorporation of a set of reverse Mitre gates or the incorporation of additional support struts. The first solution is expensive, as a whole new gate is required along with major engineering to the surrounding area. In practice, this system also requires a sensor to indicate to an operator that a reverse head is forming on the first Mitre gate, such that the reverse Mitre gate can be closed. However, due to lack of use and regular maintenance such systems are prone to mechanical failure that is only discovered at the time when the gate is most needed.
The disadvantage of employing additional support struts is two-fold. This system again requires a sensor to advise the gate operational staff that the strut has to be engaged. Secondly, it is generally deployed to the upper edge of the gate leaves, causing a pivotal action, and thus reducing the efficiency of the heel post, sill and Mitre post.
A more effective solution has been the development of a sector gate system. Although an improvement on the present form of Mitre gates, these systems are extremely expensive, as they require extensive engineering of the support structure so as to enable incorporation of the required sector recesses. Typical construction and installation costs amount to around £0.5 Million for construction of the gates but rise by around £7 Million for the development of the sector recesses.
The second major disadvantage of the Mitre gates 4 is the requirement to be constructed to a high degree of tolerance. Once installed the gate 4 has little allowance for adjustment to compensate for misalignment at the gate sills 3 or at the Mitre post. In addition, wear and tear on the heel posts and the Mitre posts results during normal opening and closing of the gate 4. This wear and tear tends to be uneven due to the load caused by the swinging weight of the gate being concentrated in particular areas and so the efficiency of the gate 4 deteriorates with use.